Which Tolkien character is known as the Witch-king of Angmar?

The Tolkien character known as the Witch-king of Angmar is the Lord of the Nazgûl.

The Witch-king is the chief of the nine Ringwraiths, or Nazgûl, servants of Sauron. Tolkien does not give him a personal name. His title comes from his rule over the northern kingdom of Angmar, which he founded in the Third Age to destroy the successor kingdoms of Arnor.

In The Lord of the Rings, he leads the attack on Minas Tirith and confronts Éowyn on the Pelennor Fields. Éowyn, aided by Merry, defeats him, fulfilling the prophecy that no living man would be able to kill him. Merry’s enchanted blade first weakens the protection that keeps the Witch-king from ordinary weapons.

The Witch-king is not Sauron himself. Sauron is the Dark Lord who controls the Ringwraiths, while the Witch-king is their most powerful captain.